Interactions between ungulates and forest insects

Sammanfattning: In forest ecosystems ungulates and insect herbivores often co-occur and utilise the same host trees. Herbivores that share a host plant can indirectly affect each other through plant induced responses. Such changes may affect plant growth, interactions with other species, and population dynamics. Studies investigating indirect interactions between taxonomically distant herbivores are lacking. Ungulates, as well as many herbivorous insect species, are considered pests and cause substantial damage to trees. Thus, interactions between these species may alter the level of damage and affect tree growth. In this thesis I investigate indirect interactions between ungulates and insects in forest ecosystems, with Scots pine (Pinus sylvestris) as the focal tree species, and the European pine sawfly (Neodiprion sertifer) as the focal insect. By combining controlled field experiments, observational studies, and modelling, I explore how ungulate browsing and grazing affect insect performance, abundance and population dynamics, and study the consequences for pine growth. I found that (i) sawfly fecundity was increased but larval survival decreased on browsed pines; (ii) browsing influenced the arthropod predator community on pines, (iii) browsing potentially could mitigate sawfly outbreaks, and that (iv) sawfly defoliation combined with ungulate browsing can, depending on the sequence and frequency, cause non-additive growth responses in pine. Lastly, I show that abundance, and diversity of and predation pressure on herbivorous insects can be influenced by habitat characteristics, which are commonly affected by ungulate grazing. I conclude that ungulates can have profound effects on forest insects and that indirect effects should not be neglected in ecological research and may have implications for e.g. forestry.